The data were analyzed using the standard LOFAR reduction packages. The radio frequency interference (RFI) were first flagged in frequency and time using the aoflagger [1]}, [2]}. As Cygnus A dominates the visibilities, no wide-field mapping was required. Therefore, the 64 frequency channels from each sub-band were averaged to a single frequency channel and to a visibility integration time of 10 s. The calibration was first performed manually for a select number of sub-bands that spanned the observed frequency range to establish a robust starting model for the source surface brightness distribution and its frequency dependence. As Cygnus A has complex structure on several angular-scales that was expected to be frequency dependent, the imaging of the data was carried out using the Multi-Scale–Multi-Frequency-Synthesis (MS-MFS) clean algorithm [3]} that is part of casa (Common Astronomy Software Applications). We found that using 5 spatial scales of width 0 (point spread function), 5, 10, 15 and 20 arcsec, and 2 Taylor terms to describe the frequency dependence provided the best results. Using a larger number of sky terms was found to increase the map noise due to the differing spectral properties of the lobes and hotspots (see below).
